Major histocompatibility class I (MHCI) has been widely studied because of its role in the positive and negative selection of CD8+ T cells, and in activating or suppressing T cell and NK cells. Neurons have been thought to express little or no MHCI, except in response to inflammatory cytokines, trauma, or functional impairment. However, recent studies have demonstrated that MHCI is expressed by some neurons whose synapses undergo activity-dependent remodeling. Notably, mice lacking 82M, TAP1 and CD3[unreadable] have deficiencies in eliminating inappropriate neuronal synaptic connections, suggesting that MHCI and a CD3^-containing receptor are involved in the selection of neuronal synaptic connections. Thus, key molecules involved in immune system function may also be involved in CNS development and plasticity. Although the lack of functional MHCI has been associated with developmental CNS aberrations, the neuroactivity of MHCI has not been directly tested. The in vitro explant system we describe herein is the first neuronal model to be responsive to exogenous MHCI. Our results show that MHCI can inhibit neuronal outgrowth in vitro. This MHCI neuroactivity is distinct from its proposed role in selecting synaptic connections, and may have parallels with MHCI's ability to inhibit T cell and NK cell function. Using this in vitro system, we can address several of the key questions in this emerging field. First, does the recognition of MHCI by CNS receptors depend on the specific MHCI allele and the presented peptide (as in the adaptive immune system), or is its recognition non-specific for the MHCI allele and presented peptide (as often occurs in the innate immune system). Second, while there is growing information on the expression pattern of MHCI in the CNS, little is known about the expression pattern of MHCI receptor(s) in the CNS. Using MHCI receptor ligands, we will for the first time, visualize classical MHCI receptors in CNS tissue sections. We will characterize the expression pattern of MHCI receptors in the developing CNS. Finally, we will determine whether different MHCI alelle/peptide combinations are recognized by different neurons. This R21 proposal addresses key questions in a new area of research, the results of which may provide important conceptual advances. Conceivably, treatments that limit the neuroinhibitory effects of MHCI could lead to new clinical approaches to mitigate neuropathological disorders. [unreadable] [unreadable]